21 March 2013

3 minute read

Introduction

Seasonal increase in bulk milk somatic cell count (SCC) is of economic importance to milk processors because of decreased production efficiency, and shelf life of products, and to producers through financial penalties. For Irish dairy herds, bulk milk SCC was highest in autumn (Berry, 2006), but this occurred during summer for UK dairy herds (Green, 2006), indicating the importance of accounting for stage of lactation in order to identify times of the year that are associated with increased cow SCC, to target further investigations.

The aim of this study was therefore to describe association between time of year, and SCC at the cow level for Irish and UK herds. In particular, to identify trends, that may be masked by the effects of stage of lactation and milk yield at the herd level.

Material and Methods

Milk recording databases were provided by the Irish Cattle Breeders Federation, and National Milk Records, UK. The study populations were 7,551 Irish dairy herds, with 10,181,545 records from 860,563 cows taken between 2005 and 2009, and 2,128 UK dairy herds, with 6,772,182 records from 474,669 cows taken between 2004 and 2006. Samples of 497 Irish, and 200 UK herds were taken at random. Four-level linear models for natural logarithm of test day SCC were developed in MLwiN 2.22, using data from herds in the first samples; random effects structure accounted for clustering of cows within herds, parities within cow, and recordings within parity. The models corrected for stage of lactation, milk yield, composition, parity, and herd size; built by backward stepwise elimination of terms from a saturated model. Factors remained in the model if p ? 0.05. Biologically plausible interactions, and herd level random were assessed. Model fit was assessed by inspection of residuals at each level for normality, and by using fixed effects to predict cow SCC for the herds used for model development. Predictions were repeated for further random samples of 493 and 200 Irish and UK herds that were not used for model development, to determine if the results could be generalised.

Results and Discussion

Following adjustment for the confounding influence of stage of lactation, associations between cow SCC and calendar month were larger and more variable from February to August, and in December, for the Irish compared to the UK herds (Figure 1). This was opposite to the trends observed in bulk milk SCC for Irish herds.

At the cow level, increased SCC and variability in SCC indicate that February to August is a key time for the occurrence of new intra-mammary infections, and some of these may become chronic, and contribute high bulk milk SCC later in the year, that is a problem for Irish dairy industry. It is therefore important to monitor rates of new infections, and cures based on individual cow SCC during summer, and take farm specific action, regardless of bulk milk SCC. However, further work is needed to demonstrate the legacy of new infections from the spring and summer, and their contribution to bulk milk SCC later in the year, as well as target rates.

Model fit was acceptable. Fixed effects from the final models were as good at predicting separate data, as they were at predicting that used for parameter estimation, indicating the results can be generalised to other Irish and UK dairy herds.

Model coefficients (and 95% confidence intervals) for calendar month (ln = natural logarithm)

Conclusions

Monitoring new intra-mammary infection rate during spring and summer is recommended as part of the national CellCheck program in Ireland.

References

Berry, D. P., B. O'Brien, E. J. O'Callaghan, K. O. Sullivan, and W. J. Meaney. (2006). J. Dairy Sci. 89:4083- 4093

Green, L. E., Y. H. Schukken, and M. J. Green. 2006. Prev. Vet. Med. 76:74-89

March 2013